Notice of Special Interest (NOSI)—Complement in Fundamental Immunology

The complement system is an evolutionarily conserved component of innate immunity that serves as a first line of defense against infection. Complement-associated molecules, known as C1 through C9, operate in a cascade of enzymatic reactions that can ultimately lead to lysis of infected cells and opsonization of pathogens to aid in phagocytic clearance. Three major pathways of complement activation have been described that are triggered independently, namely the classical, the lectin, and the alternative pathways, all of which merge at the step of C3 activation.